import json

filename='username.json'

try:
    with open(filename) as f_obj:
        username=json.load(f_obj)
except FileNotFoundError:
    username=input("What is you name?")
    with open (filename,'w') as f_obj:
        json.dump(username,f_obj)
        print("We'll rember you when you come back, "+username+"!")
else:
    print("Welcome back, "+username+"!")